Temperature compensated, frequency stabilized magnetic oscillator



3,319,185 Patented May 9, 1967 3 319,185 TEMPERATURE COMPENSATED,FREQUENCY STABILIZED MAGNETIC OSCILLATOR Wilmer C. Anderson, Greenwich,and Frank P. Rennie, Stamford, Conn., and Michael J. Ingenito, Bronx,N.Y., assignors to General Time Corporation, New York, N .Y., acorporation of Delaware Original application July 17, 1962, Ser. No.210,410, now Patent No. 3,215,951, dated Nov. 2, 1965. Divided and thisapplication Aug. 4, 1965, Ser. No. 508,176 2 Claims. (Cl. 331-413) Thisapplication is a division of applicants co-pending application 210,410,filed July 17, 1962, entitled, Temperature Compensated MagneticOscillator, now US. Patent No. 3,215,951, issued Nov. 2, 1965.

The present invention relates to magnetic oscillators and moreparticularly to means for producing constant frequency in the face ofWide temperature changes.

Magnetic oscillators employing transformers having a saturable core andwith transistor means to drive the core alternately to positive andnegative saturation are known in the art. Use of such devices has beenlimited to generation of alternating current from a D.-C. source or forother purposes where the frequency need not be precise.

It is an object of the present invention to provide a magneticoscillator in which the frequency is stably maintained as a precisevalue in the face of changes in operating conditions. More specifically,it is an object to provide a magnetic oscillator which is capable ofmaintaining a desired frequency over a wide temperature range. It isanother object of the invention to provide an improved magneticoscillator in which frequency stability is obtained simply andinexpensively employing added components which are commerciallyavailable at low cost. Consequently, it is an object to provide amagnetic oscillator which is ideally suited for use in remote equipmentor wherever a high degreeof reliability is required in the face ofdifficult operating conditions. Other objects and advantages of theinvention will become apparent upon reading the attached detaileddescription and upon reference to the drawings in which:

FIGURE 1 is an oscillator circuit embodying the pres, ent invention.

FIG. la shows the hysteresis loop characteristic of the transformer inFIG. 1.

FIG. 1b shows the change in forward voltage drop of the compensatingdiodes as a function of temperature.

modifications and equivalent arrangements included within the spirit andscope of the appended claims.

Turning now to the drawings, there is set forth in FIG. 1 a schematicdiagram of a magnetic oscillator constructed in accordance with theinvention utilizing a saturable transformer 10 having a pair of mainwindings 11, 12 and an output winding 13, all wound about a core 14. Thecore is formed of a readily saturated magnetic material having agenerally rectangular hysteresis loop as in FIG. 1a, such material beingcommercially sold by G.L. Electronics Co. under the name Orthonik TypeP1040. For driving the core into its opposite conditions of saturation,the windings 11, 12 are energized by transistors 21, 22 having base orinput circuits which are alternately energized by feedback or crossconnections including resistors 23, 24. To improve the wave form acapacitor 25 is connected across the windings 11, 12 with a resistor 26in series therewith. Using NPN transistors, positive potential isapplied to the center terminal 31 of the transformer windings forfeeding the collectors, with the emitters being returned to the negativepole via the terminal 32. The transistors are preferably of type 2N696manufactured by several manufacturers, including Fairchild, TexasInstruments, and others.

The circuit is so polarized that when conduction is initiated in one ofthe transistors the resulting induced voltage applies forward bias tothe base of such transistor causing a regenerative action. When the coresaturates, the induced voltage and impedance both diminsh. This resultsin the collector of the conducting transistor rising in potential andits base current decreasing. This rising collector voltage turns on theformer non-conducting transistor and the switching action is completedby the regenerative function mentioned above. The current in the secondtransistor now drives the core to the opposite conditon of saturation.This cycle is repeated at a rate which is directly proportional to theapplied voltage and inversely proportional to the saturation flux andnumber of turns in the transformer winding. .The voltage induced in theoutput winding 13 as the core switches from a condition of positive tonegative saturation and back again constitutes the signal.

In accordance with the present invention a diode having a negativetemperature coeflicient of resistance in the forward direction isconnected to one of the windings of the saturable transformer to providea variable shunting or loading effect thereby to maintain the frequencycon- FIG. 1c shows the variation in the resistance of a series ployed inFIGS. 1 and 2 for reducing the applied voltage upon increase intemperature.

While the invention has been described in connection Withcertain'preferred embodiments, it will be understood that the inventionis not limited to the disclosed embodiments but, on the contrary, weintend to cover the various stant upon increase in ambient temperature.More specifically, in accordance With the present invention we providean auxiliary transformer winding shunted by a pair of oppositely facingdiodes having a negative temperature coeflicient, with the winding beingso tailored and the diodes so chosen that little or no circulatingcurrent flows at temperatures up to normal ambient temperature but withprogressive increase in circulating current as the temperature isincreased beyond the normal ambient. Thus referring to FIG. 1 we haveprovided an auxiliary winding 40 coupled to the core 14 and shunted byoppositely facing diodes 41, 42 respectively. The diodes 41, 42 arepreferably of the type manufactured by Silicon Transistor Corp. andreferred to as their type STCOOS, having a forward voltage drop whichvaries with temperature as set forth in FIG. lb. Moreover, in accordancewith one of the aspects of the invention, and to provide compensation attemperatures below normal ambient, we provide, in series with thevoltage supply line a resistor having a positive temperature coefficientof resistance. In the present instance this resistor, indicated at 45,takes the form of a device commercially available under the trade nameSensistor. The Sensistor 45 has a resistance which varies withtemperature as set forth in FIG. 10. It is found that reliability andstability may be still further improved by close-coupling the resistor45 to the transformer core. To accomplish this the resistor 45 ispreferably made in the form of wire having a positive coefficient woundabout the core in a bifilar, or non-inductive, winding. A wire suitablefor this purpose is sold under the trade name Balco by the Wilbur DriverCompany.

It may be shown that in a core 14 formed of Orthonik material the fluxat saturation is not constant upon changes in temperature. Thesaturation flux, remains relatively constant for temperatures up tonormal room temperature but beyond this point the saturation fluxprogressively decreases. Since the oscillation frequency is inverselyproportional to the saturation flux, an increase in frequency withtemperature is normally experienced. However, when employing theauxiliary winding 40 and diodes 41, 42 any increase in temperaturebeyond nonmal ambient temperature produces a progressive increase incirculating current which has a loading effect upon the transformer. Itis found that such loading effect is the equivalent to making aprogressive reduction in the applied voltage. Since the oscillatingfrequency is directly proportional to applied voltage, it is found thatthe loading effect from normal ambient to a temperature of approximately100%, tends to balance out the effect of the higher temperature upon thecore material. It is found that the loading effect of the diodes 41, 42acts to compensate with a high degree of precision for the effect of thehigher temperatures upon the core material. In a practical case, asindicated in FIG. 2, it is possible, using the present teachings, tomaintain the frequency of a magnetic oscillator constant to better than0.1% over a temperature range from, say, C. to 100 C. A conventionalmagnetic oscillator, by comparison, may vary in frequency, from minus 3to plus 6 percent or more over the same temperature range.

It will be apparent, then, that we have provided a novel temperaturecompensation arrangement employing loading by diodes having a negativetemperature coefficient and voltage dropping by a resistor having apositive temperature coefficient, the net effect of which is to maintainthe frequency of oscillation constant over a wide temperature swing.This makes it possible to use the oscillator for numerous purposes wherea substantially constant frequency is desired combined withsusceptibility to miniaturized construction in remote equipment or .inapparatus requiring the highest order of reliability.

Moreover, in order to make the oscillator substantially immune to minorchanges in the voltage of the supply, we prefer to employ between thevoltage source and the oscillator a bridge circuit generally indicatedat 50 and having resistors 51-53 in three legs and a zener diode 54 inthe fourth leg. Bridges of this type are, per se, known to those skilledin the art and may be referred to as a zener bridge. It will suffice tosay that such a bridge enables closer control of voltage than ispossible using the zener alone. As a result the frequency remainssubstantially constant in the face of variations in the supply whichmay, for example, be brought about by the gradual exhaustion of supplybatteries when the device is employed in remote locations as, forexample, on satellites or the like.

While the invention has been discussed above in connection withoppositely connected diodes for loading transformer winding 40, theinvention is not limited thereto but includes other auxiliary load meansin which the loading effect increases upon increase in temperature tothe extent necessary to effect compensation for the effect oftemperature upon the core material and associated transistors. Thus asshown in FIG. 1d, the auxiliary winding, here indicated at a has a shuntload in the form of a thermistor 41a having a negative temperaturecoefficient of resistance, i.e., a negative coefficient of forward drop.In order to tailor or vary the effect of the thermistor, the thermistorpreferably is shunted by a conventional resistor 42a. It will beapparent, also, to one skilled in the art that the magnitude of thethermistor effect may be varied by varying -a series resistor in thethermistor leg. The precise value of the resistors 41a, 42a depends uponthe degree of compensation desired which in turn depends upon theparticular characteristics of the core material and the transistorsemployed. In a practical case, compensation has been brought about byusing a thermsitor having a resistance of 1000 ohms at normal ambienttemperature and a nominal negative temperature coefficient of 4.4%shunted by a resistor 42a of 1000 ohms.

The invention is not limited to the embodiment described above butincludes use of diodes having a negative temperature coefficient ofresistance connected to other windings of the saturable transformer forfrequency stabilization purposes. Thus referring to FIG. 3, there isprovided a saturable transformer 60 having windings 61, 62, 63, 64. Thewindings 61, 62 correspond to the windings 11, 12 in the previousversion; the windings 63, 64 constitute auxiliary windings employed toexcite the base or input terminals of the transistors. The transformerhas a core 65, as before, formed of magnetic material which is easilysaturated and which is characterized by a generally rectangularhysteresis loop. The transistors indicated at 71, 72 may be of the sametype as transistors 21, 22 of the previous embodiment. For energizingthe base or input terminal of the transistor 71 it is connected to thecentral point of a voltage divider formed of resistors 73, 74.Corresponding resistors 75, 76 are associated with the base of thetransistor 72. Proper proportioning of the resistors determines the biason the transistor base and determines the region of the transistorcharacteristic over which operation takes place. Stabilizar tion isprovided by low value degeneration resistors 77,

78 in the emitter circuits while damping is provided by shuntingresistors 81, 82 across the transformer windings 61, 62 respectively.For maintaining the voltage applied to the collector terminal 84substantially constant in the face of changes in the supply, a zener 85is employed, shunted to ground, and a dropping resistor 86 is providedin series with the supply terminal, indicated at 87.

In the operation of the circuit described above, application of voltagecauses both of the transistors to tend to conduct but because of slightinherent unbalance in the circuit one will normally tend to conduct moreheavily than the other. Conduction in the predominating transistorinduces a voltage in the associated control winding which is in such adirection as to bias such transistor in the forward direction so thatthe predominating transistor tends to conduct more heavily while theremaining transistor tends to become non-conductive. When saturation isreached, the rate of change of flux decreases, hence the induced voltagedecreases. By transformer action the bias voltage on the conductingtransistor also diminishes, hence the current in this transistordecreases so that the transistor becomes non-conducting. The decayingcurrent induces a voltage across the bias winding of the off transistorin the direction to turn it on. The conduction is in a direction toincrease the induced forward bias so that the second transistor conductscurrent heavily to drive the core into the condition of opposite, ornegative, saturation. When saturation is reached, and slightly exceeded,the resulting reduction in current reduces the bias of the thenconducting transistor but increases the forward bias on the oppositetransistor so that the core is driven back to a condition of positivesaturation. This oscillation continues, first one of the transistorsconducting and then the other, at a frequency which is, as in theearlier embodiment, determined by the transformer geometry and theapplied volt-age.

In carrying out the present invention diodes having a negativetemperature coefficient of resistance are connected to the windings 63,64 which control the base circuits of the transistors 71, 72respectively. Such diodes, indicated at 91, 92 are, in the presentcircuit, effectively in parallell with the resistors 73, 75 previouslyreferred to. Each diode, during the conductive portion of the cycle,acts to reduce the resistance in the associated control circuitconsistingof the control winding and the baseemitter junction of thetransistor. However, the effect at all temperatures is not the same.Thus, there will be a lower effective resistance, and hence greaterconductivity, the higher the temperature. The windings 63, 64 beingheavily shunted thus tend to oppose any sudden change or collapse offlux and hence tend to increase the pulse width. Moreover, the increasein current flow in the control windings, in other words the increase inloading effect, is mirrored in the amount of current flowing in the maintransformer windings. The net effect is to compensate for the tendencyof the circuit to increase in frequency upon increase in temperature, asregards the temperature characteristics of the core material. It isfound that the circuit shown in FIG. 3 is capable of maintaining aconstant frequency to almost the same degree as the circuit of FIG. 1.If desired, a series resistor 95 may be interposed in the supply legcorresponding to the resistor 45 in FIG. 1.

By way of example and to assist in putting the present invention to use,it will be helpful to specify the circuit constants which have beenemployed in a practical case. Thus in both FIGS. 1 and 3 the core mayconsist of a ribbon formed of Orthonik approximately 4" in width and0.00025" in thickness wound about a A" bobbin to a total number of 22turns. For a frequency of K cycles per second, the windings 11, 12 inFIG. 1 may be formed of 105 turns of #33 wire. The output winding 13 maybe formed of 55 turns and the auxiliary winding 40 of 10 turns. In thecase of the embodiment shown in FIG. 3 the main windings 61, 62 may beformed of 105 turns of #38 wire and the windings 63, 64 of 33 turns. Forfrequency adjustment, a slightly larger number of turns may be employedwith the turns being successively removed one by one on the mainwindings until the desired frequency is achieved. The remaining circuitconstants in the preferred embodiments, FIGS. 1 and 3, are as follows:

21, 22 Transistors Type 2N7l7. 23, 24 3.9K ohms.

25 470 micromicrofarads. 26 680 ohms.

41, 42 STCOOS.

45 68 ohm Sensistor.

51 47 ohms.

52 1.8K ohms.

53 220 ohms.

54 IN825A.

71, 72 Transistors Type 2N717. 73, 75 1000 ohms.

74, 76 8200 ohms.

77, 78 4.7 ohms. I

81, 82 4700 ohms.

85 Diode Type IN825A.

86 270 ohms-330 ohms.

91, 92 Diodes Type STCOOS.

For lower frequencies, a correspondingly greater number of turns andlarger cores may be used.

While temperature compensation is effected, in part, in FIG. 1 by use ofa series resistor 45 having a positive temperature coefficient ofresistance, it will be understood that the invention is not limitedthereto but includes use of control elements in a parallel leg of thecircuit and having a negative coefiicient of resistance to bring aboutthe same result. For example, referring to FIG. 4, there are provided,in parallel with the oscillator circuit 100, one or more zener diodes101, 102 having a negative coefficient of resistance, as, for exampleType IN747.

Current is supplied to the parallel circuit via a dropping resistor 103fed by a terminal 105 from a battery or similar source. In operation anincrease in temperature acts to lower the point of the zener conduction.This lowers the voltage maintained at the input terminal 106 of theoscillator 100. Such lower voltage has the effect of reducing thefrequency thereby to compensate for the decrease in saturation flux inthe core of the saturable transformer which, uncompensated, would tendto produce increase in frequency of oscillation. It may be necessary toemploy more than one of the zener diodes in series since such diodes arecurrently available only in relatively low voltage ratings.

The same regulatory effect can be achieved by the arrangement shown inFIG. 5. Here the oscillator indicated generally at 110 has a parallelcircuit which consists of a zener diode 111 in series with non-zenerdiodes having a negative temperature coefficient and indicated at 112,113. The zener diode may be of Type IN825A and the remaining diodes ofType STCOOS". Current is supplied to the parallel circuit by a droppingresistor 114 from a source 115. The reduction in voltage drop across thediodes 112, 113 which tends to occur at higher temperatures reduces thevoltage maintained at terminal 116. As stated above, this compensatesfor the effect of temperature upon the core material with the resultthat the frequency is maintained more nearly constant.

Stability has been achieved by use of auxiliary circuit components whichare inexpensive and readily available. Constant frequency is maintainedin spite of shock and vibration and the resistance to environmentalchanges may be further reduced by potting. Since all of the componentsare inherently small, the circuits are ideally suited to miniaturizationas building blocks in more complex apparatus, wherever a stableoscillator is required. The frequency may be varied over wide limitssimply by varying the number of turns and size of core in thetransformer.

We claim as our invention:

1. In a saturable core magnetic oscillator comprising a saturabletransformer having a saturable core formed of material characterized bya generally rectangular hysteresis characteristic as well as by areduction in permeability upon increase in ambient temperature, saidtransformer having a pair of saturating windings and a pair oftriggering windings, a source of voltage, first and second switchesinterposed between the voltage source and the transformer windings, eachof said switches having an input circuit and an output circuit connectedto the triggering and saturating windings respectively so that the coreis driven alternatively between its positive and negative conditions ofsaturation at a regular oscillatory rate, means for stabilizing thefrequency of said oscillator under varying temperature conditions, saidmeans comprising first and second diodes interposed in the inputcircuits of the respective switches and of the type having a forwardvoltage drop which decreases substantially linearly with increase inambient temperature thereby to increase the load upon the respectivetriggering windings progressively at elevated temperatures formaintaining a substantially constant frequency of oscillation over atemperature range.

2. In a saturable core magnetic oscillator comprising a saturabletransformer having a saturable core formed of material characterized bya generally rectangular hysteresis characteristic as well as by areduction in permeability upon increase in ambient temperature, saidtransformer having a pair of saturating windings and a pair oftriggering windings, a source of voltage, first and second transistorsinterposed between the voltage source and the transformer windings, saidtransistors having their base and collector circuits connected to thetriggering and saturating windings respectively so that the core isdriven alternatively between its positive and negative conditions ofsaturation at a regular oscillatory rate, means for stabilizing thefrequency of said oscillator under varying temperature conditions, saidmeans comprising first and second diodes interposed in the base circuitsof the respective transistors and of the type having a forward voltagedrop which decreases with increase in ambient temperature, anddegenerating resistors in series with the emitters of the respectivetransistors for maintaining a substantially constant frequency ofoscillation over a temperature range.

References Cited by the Examiner UNITED STATES PATENTS 2,875,351 2/1959Collins 3311l3 ROY LAKE, Primary Examiner.

I. KOMINSKI, Examiner)

1. IN A SATURABLE CORE MAGNETIC OSCILLATOR COMPRISING A SATURABLETRANSFORMER HAVING A SATURABLE CORE FORMED OF MATERIAL CHARACTERIZED BYA GENERALLY RECTANGULAR HYSTERESIS CHARACTERISTIC AS WELL AS BY AREDUCTION IN PERMEABILITY UPON INCREASE IN AMBIENT TEMPERATURE, SAIDTRANSFORMER HAVING A PAIR OF SATURATING WINDINGS AND A PAIR OFTRIGGERING WINDINGS, A SOURCE OF VOLTAGE, FIRST AND SECOND SWITCHESINTERPOSED BETWEEN THE VOLTAGE SOURCE AND THE TRANSFORMER WINDINGS, EACHOF SAID SWITCHES HAVING AN INPUT CIRCUIT AND AN OUTPUT CIRCUIT CONNECTEDTO THE TRIGGERING AND SATURATING WINDINGS RESPECTIVELY SO THAT THE COREIS DRIVEN ALTERNATIVELY BETWEEN ITS POSITIVE AND NEGATIVE CONDITIONS OFSATURATION AT A REGULAR OSCILLATORY RATE, MEANS FOR STABILIZING THEFREQUENCY OF SAID OSCILLATOR